1. Field of the Invention
The invention relates to a device for applying, in use, electromagnetic microwave radiation in a plasma cavity, which plasma cavity is present within a housing, being substantially cylindrically symmetric about a first axis, and which comprises a cylindrical wall provided with a circumferential slit, said plasma cavity being in communication, via said slit, with a first end of an elongated microwave guide having a longitudinally extending second axis, said device being in communication in use with microwave generating means via the other end of said microwave guide, in which the electromagnetic microwave radiation being generated may comprise several modes.
The invention also relates to an apparatus for depositing, by means of a Plasma Chemical Vapour Deposition process, one or more glass layers on the inner side of a hollow glass tube disposed in the plasma cavity of the device according to the invention.
The invention furthermore relates to a method for depositing, by means of a Plasma Chemical Vapour Deposition process, one or more glass layers on the inner side of a hollow glass tube disposed in the plasma cavity of the device according to the invention.
2. Discussion of the Background
Such a device, which is also referred to by the technical term microwave applicator, the slit being called a radial wave duct, can be used in particular with a sufra guide or a sufraton for generating a plasma in the plasma cavity. Another practical application of a microwave applicator according to the introduction is disclosed in International patent application PCT/EP98/07798, published under No. WO 99/35304 in the name of the present applicant.
WO 99/35304 shows a PCVD (Plasma Chemical Vapour Deposition) apparatus, in which a hollow cylindrical glass tube is accommodated in the plasma cavity of the microwave applicator, through which tube a gaseous mixture comprising O2, SiCl4 and GeCl2 for example, is passed. The microwave radiation applied in the slit by the microwave applicator locally generates a plasma in the hollow glass tube. By moving the microwave applicator back and forth with respect to the glass tube—possibly with simultaneously rotation of the tube—glass layers, whether or not of different composition, can be deposited on the inner side of the cylindrical glass tube by means of the plasma. In this way a so-called hollow preform having one or more glass layers deposited on the inner side thereof is obtained. The preform is subjected to a collapsing treatment so as to obtain a massive rod, from which an optical fibre is subsequently drawn, which fibre is intended for telecommunication purposes, for example.
In the PCVD application, the microwave radiation being used or being applied in the slit has a frequency of 890 MHz or 2.45 GHz and may comprise several so-called modes. The term modes as used herein is understood to mean the propagating modes in the radial propagation direction as well as the modes that have just been cut off. The propagating modes comprises a number of zero-axis crossings in the electrical field in both directions perpendicularly to the radial propagation direction.
If the width of the radial wave slit amounts less than half the wavelength of the microwave radiation being used, only one propagating mode will occur if the inner circumference of the radial wave slit has a length equal to or smaller than the wavelength of the microwave radiation being used, whilst three propagating modes will occur if the inner circumference of the radial wave slit has a length of 1-2 wavelengths of the microwave radiation being used.
It is very difficult with the current applications to determine which modes the microwave radiation being applied in the slit (radial wave duct) comprises. If the microwave radiation applied in the plasma cavity or the radial wave duct comprises several modes, the geometric properties of the plasma generated in the plasma cavity will not be known very exactly. If said modes exhibit the same electromagnetic field distribution, seen in the direction of rotation relative to the first axis, the geometric property of the applied microwave radiation and thus also of the generated plasma in the aforesaid direction of rotation will be clearly defined. Said plasma properties depend, among other things, on the excitation ratio of the modes, and a small change in the environment (temperature, pressure, etc) may lead to a different excitation ratio, causing the geometric properties of the plasma to change. All this may lead to an undesirable geometry of the plasma, amongst other things, or cause the plasma to become unstable or become extinguished altogether.
In particular in the case of PCVD applications such phenomena affect the composition and the structure of the glass layers being deposited on the inner side of the hollow glass tube, as well as the final quality of the optical fibre drawn from a preform thus obtained.
One solution for the problem of the modes of the microwave radiation applied in the plasma cavity and the unpredictable properties of the generated plasma can be found in the maximum allowable internal diameter of the radial wave duct, in which the inner circumference is smaller than or equal to the wavelength of the microwave radiation being used, this in order to prevent the occurrence of several modes. However, said maximum constructional dimension will also limit the maximum allowable external diameter of the preform to be manufactured in that case, as well as the length of the optical fibre drawn from said preform.